Pliocene–Pleistocene boundary determination in hemipelagic sediment from the Ulleung Basin (East Sea, offshore Korea) inferred from terrigenous and marine palynofloras

Abstract

Palynofloral analysis was performed for the first time on sediment from the Ulleung Basin (East Sea, offshore Korea) to locate the Pliocene–Pleistocene boundary, which is very important in determining the depositional age of a stratigraphic unit that contains methane hydrate. Cores from the drill sites Ulleung Basin Gas Hydrate 1–9 (UBGH1–9) and Ulleung Basin Gas Hydrate 1–10 (UBGH1–10) in the Ulleung Basin produced abundant to common pollen grains and organic-walled dinoflagellate cysts. Age-diagnostic palynomorphs were present in certain intervals: 120–175mbsf at site UBGH1–9 and 170–205mbsf at site UBGH1–10. The biostratigraphically meaningful taxa were the pollen genera Carya, Liquidambar, and Fagus and the dinoflagellate cysts Capillicysta fusca, Filisphaera filifera subsp. pilosa, and Selenopemphix quanta. The latest stratigraphic occurrence of these pollen taxa in northeast Asia is the late Pliocene, and that of the dinoflagellate cysts is regarded as the late Pliocene in all aquatic areas, especially in the Pacific. The last appearance datum (LAD) of the age indicators in the two cores studied suggests that the Pliocene–Pleistocene boundary is at 120mbsf at site UBGH1–9 and 170mbsf at site UBGH1–10.